1. Field of the Invention
The present invention relates to a semiconductor device and a display device. In this specification, a semiconductor device refers to a semiconductor element itself or a device including a semiconductor element. As an example of such a semiconductor element, for example, a transistor (such as a thin film transistor) can be given. In addition, a semiconductor device also refers to a display device such as a liquid crystal display device.
2. Description of the Related Art
In recent years, display devices using liquid crystal panels and display devices using organic EL panels have been under active development. These display devices are broadly classified into display devices in which only transistors for pixel control (pixel transistors) are formed over a substrate and scanning circuits (driver circuits) are included in peripheral ICs and display devices in which scanning circuits are formed over the same substrate as pixel transistors.
Display devices in which driver circuits are integrated with pixel transistors are more effective for reduction in frame width of the display devices or cost of the peripheral ICs. However, transistors used in the driver circuits are required to have better electrical characteristics (e.g., field-effect mobility (μFE) or threshold voltage) than the pixel transistors.
Although a silicon-based semiconductor material has been widely known as a material for a semiconductor thin film applicable to transistors, an oxide semiconductor is attracting attention as another notable material (e.g., Patent Documents 1 and 2). For example, attention is drawn to a transistor using as a semiconductor thin film an amorphous oxide which contains indium (In), gallium (Ga), and zinc (Zn) and has an electron carrier concentration of less than 1018/cm3.
Transistors using oxide semiconductors for semiconductor layers have higher field-effect mobility than transistors using amorphous silicon which is a silicon-based semiconductor material for semiconductor layers. Hence, the transistors using oxide semiconductors can operate at high speed and be suitably used for the display devices in which driver circuits are integrated with pixel transistors. Besides, a manufacturing process for such transistors using oxide semiconductors is easier than a manufacturing process for transistors using polycrystalline silicon for semiconductor layers.
However, a problem of the transistors using oxide semiconductors for semiconductor layers is that entry of impurities such as hydrogen or moisture into the oxide semiconductors generates carriers and changes the threshold voltage which is one of electrical characteristics of the transistors.